The overall objective of our research is the characterization of the inorganic anion permeability pathways of the Ehrlich mouse ascites carcinoma cell. The kinetic characteristics of chloride, sulfate and inorganic phosphate transport across the plasma membrane suggests that in order for these anions to enter the cell they must first interact with specific membrane carrier systems. Previous studies have provided evidence consistent with the assertion that chloride and sulfate utilize a single carrier system possessing two reactive sites while phosphate utilizes an entirely separate system. Recent studies have shown that chloride may use three separate permeability pathways: nonmediated diffusion, cotransport with potassium and sodium, and mediated selfexchange through the anion transporter. Phosphate, on the other hand, is transported by a system that requires the presence of sodium in the extracellular medium. During the past year our efforts have been directed toward characterizing this sodium-dependent phosphate transport mechanism. Our results indicate that in normal physiological medium phosphate transport exhibits saturation kinetics with the Km for phosphate equal to 0.12 mM. When the extracellular sodium concentration is reduced, phosphate transport decreases and, in sodium-free medium, transport is only 10-15% of that measured at 150 mM sodium. Furthermore, the intracellular phosphate concentration, which normally is in the range of 12-14 mmoles/Kg cell water, decreases by 30 to 40% in the absence of sodium. This indicates that continuous sodium-phosphate cotransport is necessary for maintenance of intracellular phosphate. Evidence also has been obtained suggesting that monovalent phosphate is the predominant, if not the exclusive, species transported and that a decrease in the intracellular pH results in inhibition of phosphate transport. Since pH also affects chloride transport, in the next year of this project we plan to investigate the relationship between the transmembrane proton and anion transport.